2-2.1 Polynucleotide phosphorylase:

• Polynucleotide phosphorylase was first discovered from extracts of Azotobacter agile by Grunberg-Manago and Ochoa.

• Polynucleotide phosphorylase (PNPase) catalyzes the synthesis of long chain polyribonucleotides (RNA) in 5' to 3' direction from nucleotide diphosphates as precursors and reversibly catalyzes phosphorolytic cleavage of polyribonucleotides in 3' to 5' direction with a release of orthophosphate in presence of inorganic phosphate.

• PNPase is a bifunctional enzyme and functions in mRNA processing and degradation inside the cell.

• Structural and physiochemical studies in enzymes showed that it is formed of subunits. The arrangements of the subunits may vary from species to species which would alter their properties.

• These enzyme can catalyze not only the synthesis of RNA from the mixtures of naturally occurring ribonucleoside diphosphates, but also that of non-naturally occurring polyribonucleotides

2-2.1.1 Mechanism of action:

As mentioned earlier, polynucleotide phosphorylase is a bifunctional enzyme. The mechanism of action of this enzyme can be represented by following reactions:

Fig 2-2.1: Schematic representation of the role of PNPase in poly(A) tail metabolism in E. coli .